A semiconductor layout is created manually or automatically using layout software. The layout data are pre-processed and written on a mask. For example, the mask incorporates a chrome layer on glass where the chrome is patterned according to the layout data. The mask is usually repeatedly used to pattern chips located on a substrate, for example, a silicon wafer, with a photographic process called lithography.
Typically, this patterning includes the following steps:                Photographically transfer the pattern into a light-sensitive photoresist and develop the photoresist such that a physical resist pattern is created on the substrate.        
Transfer the photoresist pattern into the layers on the substrate by etching or ion implantation.
Remove the photoresist.
Typically, this process is repeated several times for respective layers of the layout (such as metal conductors, contact holes).